BACKGROUND: AIDS reporting in Switzerland is mandatory by law. For the first time since the beginning of the epidemic, health authorities observed a decline in AIDS surveillance reports, from 545 new cases in 1995 to 459 in 1996, a decrease of 86 cases. OBJECTIVE: To examine the extent to which this decline in AIDS surveillance reports is attributable to the introduction of antiretroviral combination therapy. Patients and setting: Swiss HIV Cohort Study, a multicentre cohort of adult HIV-infected patients with national coverage. Participants at risk of developing a first AIDS-defining event, defined as persons with a CD4 cell count below 200 x 10(6)/l or in clinical stage B, were studied. METHODS: A parametric statistical survival model was used to estimate the number of AIDS cases expected in the absence of combination therapies. Taking reporting delays into account, the effect of the introduction of combination therapies in the cohort on national AIDS reports was estimated from the difference between the number of expected and observed cases. RESULTS: A total of 4915 participants contributed 10755 person-years and 2366 initial AIDS events. Between 1990 and 1994, about 35% received antiretroviral therapy, predominantly monotherapy. In 1995 and 1996, the prevalence of antiretroviral therapy increased steadily due to the introduction of dual and triple combinations. The mid-year prevalence of use of combination therapies was 6% in 1994, 13% in 1995, and 48% in 1996. The difference between expected and observed cases per half-year increased from 12 in the first 6 months of 1994 to 69 in the second half of 1996. Taking reporting delays into account, we estimated that 43 (95% confidence interval, 17-69) of the deficit of 86 reports could be attributed to combination therapies. CONCLUSION: The introduction of antiretroviral combination therapies in a single cohort study explained 50% of the decline in national AIDS surveillance reports. Monitoring the use and effectiveness of antiretroviral therapy must become an important component of AIDS surveillance systems.